A few long disclaimers:
discipline on a mage with crossclassed disc->
16 cc'd
+5 skillfocus
+6 strength modifier, 27 raw
gear: 11 slots, +2 each, 22 baseline
piker's rings: 3 disc each, +2 total over baseline. 24.
watcher's discipline amulet: requires lawful. 5 discipline, +3 over baseline. 27.
sergeant's cloak: 3 discipline, +1 over baseline. 28.
55 discipline. This shifts with base STR (higher if base STR is higher than 10, lower if base STR is lower than 10.)
And, i may have skipped something still.
SIDE NOTE: Earth elementalist. They get +10 free discipline, shifting the discipline from 55 to 65.
RE: knockdown.
The formula to KD is
Total attack roll (so, 1d20 + AB) + 4*size diff
VS
victim's discipline score + 1d20.
Additional modifiers: Improved knockdown makes you count as one size bigger, negating the -4 penalty to attack for the formula.
Assuming 50 AB:
Maximum attack roll is 70 (very rare occurrance)
KD check DC: 70 -4 (KD penalty) +4 (IKD upgrade)
70.
Meaning: To become actually, positively, 100% immune to knockdown coming from someone without true strike, with 50 AB, you'd need 69 discipline. Since i believe tie goes in favor of the victim.
Now, assuming 5 of that AB comes from weapon enhancement for example, truestrike would give +15 AB over that.
Effectively moving the maximum possible KD check DC from 70 to 85.
Requiring 84 total discipline to be immune to truestrike KD, coming from someone of the same size as you, with this AB bonus.
Any size difference between you and your opponent shifts the discipline requirement here by 4.
And yes, i KNOW nat 20 KD are rare. you don't have to tell me that.
And yes, i also know that truestrike KD is, in a normal fight, in wilderness, with a lot of room to run around, hard to pull off.
RE: DC spells
I mentioned save DC spells because the effect is functionally identical, or at least very similar.
When you fail a save vs a DC spell, you're either CC'd for a long Snuggybear time and have to burn a pray, or you just outright die.
When you fail a discipline check vs KD, you take some severe damage, which oftentimes forces you to burn a pray to survive, or you just die to the damage output, especially if the attacker was a weaponmaster. A full, whole round of attacks vs a flatfooted opponent is lethal.
The difference here is of course that spells have range. But spells also have immunity wards that give you total, 100% immunity to these spells that instantly kill you. Please bear that in mind. There's no sippy to make you totally immune to knockdown.